Save impression method does not validate histogramIndex against maximum histogram size

IIUC it should probably be checked against [the same implementation-defined maximum used in `measureConversion`](https://w3c.github.io/ppa/#ref-for-dom-privateattributionconversionoptions-histogramsize%E2%91%A0) in order to avoid storing impressions that can *never* be used to fill a histogram ([Step 5 here](https://w3c.github.io/ppa/#fill-a-histogram-with-last-touch-attribution)).